Wireless communications around the world are moving toward CDMA and other multi-signature systems in the third generation. The rapid increase in the number of users of mobile telephones, personal communication services, etc., places challenging demands on future wireless services. Viewed as the generic next generation commercial multiplexing scheme, CDMA offers the promises of efficient use of the available bandwidth, inherent resistance to interference and adaptability to variable traffic patterns.
In some CDMA applications where multi-path delays are not negligible, it is normally difficult to maintain the low cross-correlation among subscribers' signature signals since communication channels are subject to frequency-selective fading. Signal reception using conventional matched filters may suffer severe performance degradation due to mutual interference, especially in a near-far situation (e.g., weak CDMA signals can be overwhelmed by strong power signals in the same system). A class of multi-signature receivers for demodulating CDMA signals has been developed that tries to mitigate the effect of multiple signature signal interference and background noise. These include optimal multi-user receivers, minimum mean-squared error (“MMSE”) receivers, decorrelators, and matched filter (“MF”) receivers. See generally, S. Verdu, Multiuser Detection, Cambridge University Press, 1998, which is hereby incorporated by reference.
Both the optimal receiver and the MMSE receiver require knowledge of the channel parameters, namely the noise level and the received amplitudes of the users' signals. In addition, the optimal receiver is nonlinear and requires knowledge of the probability density function of the noise, making it very complicated to implement. As a result, most commercial users do not use optimal receivers, instead relying on sub-optimal receivers. This use of sub-optimal receivers results in inferior performance in multi-user CDMA environments.
MF and decorrelator receivers, on the other hand, require knowledge of only the signature signals. Although MF receivers optimally compensate for white noise, they do not exploit the structure of multi-user interference. See R. Lupas and S. Verdu, “Linear Multiuser Detectors for Synchronous Code-Division Multiple-Access Channels,” IEEE Trans. on Info. Theory, vol. 35, pp. 123-136, January 1989; and D. N. C. Tse and S. V. Hanly, “Linear Multiuser Receivers: Effective Interference, Effective Bandwidth and User Capacity,” IEEE Trans. on Info. Theory, vol. 45, pp. 641-657, March 1999. Decorrelator receivers, conversely, optimally reject the multi-user interference but do not account for the white noise. There is a need, therefore, for a receiver that mitigates both the effect of multi-user interference and white noise, while requiring knowledge of only the signature signals. Such a receiver will often achieve acceptable performance irrespective of the white noise distribution, the channel gains, and the channel signal-to-noise-ratio.